The present invention relates to a hydraulic drive system including a variable displacement hydraulic pump, and more particularly to a hydraulic drive system in which load sensing control is performed to control the displacement of a hydraulic pump such that the difference pressure between a delivery pressure of a hydraulic pump and a maximum load pressure among a plurality of actuators is maintained at a setting value.
As load sensing techniques for controlling the displacement of a hydraulic pump so as to maintain the difference pressure between a delivery pressure of the hydraulic pump and a maximum load pressure among a plurality of actuators at a setting value, there are known a pump displacement control unit disclosed in JP,A 5-99126 and a hydraulic drive system disclosed in JP,A 10-196604.
The pump displacement control unit disclosed in JP,A 5-99126 comprises a servo piston for tilting a swash plate of a variable displacement hydraulic pump, and a tilting control unit for supplying a pump delivery pressure to a servo piston in accordance with a differential pressure xcex94PLS between a delivery pressure Ps of a hydraulic pump and a load pressure PLS of an actuator, which is driven by the hydraulic pump, and for maintaining the differential pressure xcex94PLS at a setting value xcex94PLSref, thereby performing displacement control. The pump displacement control unit further comprises a fixed displacement hydraulic pump driven by an engine along with the variable displacement hydraulic pump, a throttle disposed in a delivery path of the fixed displacement hydraulic pump, and means for changing the setting value xcex94PLSref in the tilting control unit in accordance with a differential pressure xcex94Pp across the throttle. Then, the setting value xcex94PLSref of the tilting control unit is changed by detecting an engine revolution speed based on change of the differential pressure across the throttle disposed in the delivery path of the fixed displacement hydraulic pump.
The hydraulic drive system disclosed in JP,A 10-196604 is constructed by providing, in a hydraulic circuit disclosed in JP,A 5-99126, a plurality of pressure compensating valves for controlling differential pressures across a plurality of flow control valves to be held at the same differential pressure between a pump delivery pressure and a maximum load pressure, and by forming the throttle disposed in the delivery path of the fixed displacement hydraulic pump as a variable throttle that has a larger opening area when an engine revolution speed is in a range nearer to a rated revolution speed than when it is in a range nearer to a minimum revolution speed. With such an arrangement, when the engine revolution speed is set to a lower value, a target compensated differential pressure for each of the pressure compensating valves is reduced to a larger extent. As a result, actuator speed is slowed down and good fine operability can be achieved.
In the prior art, as described above, a fixed throttle or a flow detecting valve (variable throttle) is disposed in the delivery path of the fixed displacement hydraulic pump, and the setting value xcex94PLSref in the load sensing control is changed in accordance with the differential pressure across either throttle. The setting value xcex94PLSref is thereby reduced depending on the engine revolution speed so as to slow down the actuator speed.
The above-described prior art, however, has a problem in that when a speed change width required for an actuator is large, the prior art is not adaptable for such a requirement.
For example, excavation-and-loading work is one of ordinary work carried out by a hydraulic excavator. In that work, after excavation, scooped earth and sand are released and loaded on a track bed by raising a boom while a swing body is driven to swing. Also, crane work has recently been carried out using a hydraulic excavator in many cases. In the crane work, a load is hung at a fore end of a front operating mechanism and is slowly swung. The swing speed required in the excavation-and-loading work differs greatly from that required in the crane work. When one hydraulic excavator is employed to carry out both the excavation-and-loading work and the crane work, a change width of the swing speed exceeds the range obtainable in the above-described prior art through adjustment of the engine revolution speed, and the above-described prior art is not adaptable for such a large change width of the demanded actuator speed.
Even if using an electric motor as a prime mover can provide a sufficiently large width in adjustment of the revolution speed through inverter control and make a system adaptable for a large change width of the demanded actuator speed, an operator feels somewhat different from the operation of a conventional system in setting the revolution speed of the prime mover for adjustment of the actuator speed.
More specifically, when an operator reduces the revolution speed of the prime mover for fine operation in ordinary excavation work, the revolution speed of the prime mover must be adjusted while paying attention to such a point that the actuator speed will not slow down to a level unsuitable for carrying out ordinary excavation work. This imposes an excessive burden on the operator.
An object of the present invention is to provide a hydraulic drive system in which a target differential pressure in load sensing control can be changed depending on the revolution speed of a prime mover, and even when a change width of the demanded actuator speed exceeds the range adjustable with the revolution speed of the prime mover, the system is adaptable for such a change width and can realize the respective demanded actuator speeds.
(1) To achieve the above object, according to the present invention, there is provided a hydraulic drive system comprising a prime mover; a variable displacement hydraulic pump driven by the prime mover; a plurality of actuators driven by a hydraulic fluid delivered from the hydraulic pump; a plurality of flow control valves for controlling flow rates of the hydraulic fluid supplied from the hydraulic pump to the plurality of actuators; a plurality of pressure compensating valves for controlling differential pressures across the plurality of flow control valves depending on a differential pressure between a delivery rate of the hydraulic pump and a maximum load pressure among the plurality of actuators; pump displacement control means for controlling a displacement of the hydraulic pump and maintaining the differential pressure between the delivery rate of the hydraulic pump and the maximum load pressure among the plurality of actuators at a setting value; and a fixed displacement hydraulic pump driven by the prime mover along with the variable displacement hydraulic pump; the pump displacement control means including throttle means provided in a delivery line of the fixed displacement hydraulic pump, detecting change in revolution speed of the prime mover based on change in differential pressure across the throttle means, and changing the setting value depending on the revolution speed of the prime mover; wherein the hydraulic drive system further comprises a selector valve connected to the throttle means in parallel and being operable to shift between a fully closed position and a throttle position.
With the provision of the selector valve in parallel to the throttle means, when the selector valve is in the fully closed position, the throttle means functions solely and the setting value in pump displacement control (target differential pressure in load sensing control) can be adjusted depending on the revolution speed of the prime mover in the same manner as that conventionally performed. When the selector valve is shifted to the throttle position, the hydraulic fluid from the fixed displacement hydraulic pump is distributed to the throttle means and the selector valve, whereupon the flow rate of the hydraulic fluid passing through the throttle means is reduced and the differential pressure across the throttle means is also reduced. As a result, even at the same revolution speed of the prime mover, the setting value becomes smaller than that resulting when the selector valve is in the fully closed position. This reduces the differential pressure across the flow control valve controlled by the pressure compensating valve. Hence, the flow rate of the hydraulic fluid supplied to the actuator is reduced and the actuator speed is slowed down.
Thus, the target differential pressure in the load sensing control can be changed depending on the revolution speed of the prime mover. Also, even when a change width of the demanded actuator speed exceeds the range adjustable with the revolution speed of the prime mover, the system is adaptable for such a large change width and can realize the respective demanded actuator speeds.
(2) In above (1), preferably, the hydraulic drive system further comprises manual operating means for shifting the selector valve between the fully closed position and the throttle position.
With that feature, it is possible to shift the selector valve and change the actuator speed in accordance with the operator""s intention.
(3) In above (1), preferably, the hydraulic drive system further comprises manual operating means operated by an operator; and switching means for shifting the selector valve between the fully closed position and the throttle position in response to an operation of the manual operating means.
That feature also makes it possible to shift the selector valve and change the actuator speed in accordance with the operator""s intention.
(4) In above (3), preferably, the switching means are electrically and hydraulically operated.
With that feature, the selector valve can be shifted in a hydraulic way.
(5) In above (3), the switching means may be electrically operated.
With that feature, the selector valve can be shifted in an electrical way.
(6) Further, in above (1), the selector valve is able to change an opening area continuously when the selector valve is in the throttle position.
With that feature, the actuator speed can be freely adjusted in accordance with the operator""s preference.